This invention relates to helicopters. More particularly this invention relates to directional thrusters used to impart lateral forces on the tail boom of a helicopter for antitorque and yaw control. Still more particularly, this invention relates to rotating thruster assemblies that shift air jet flow between the left and right sides of the tail boom.
Tail rotors have been the dominant means for providing yaw control in helicopters as well as overcoming the biasing torque produced by the main rotor. However, tail rotors contribute to over 15% of all helicopter accidents, largely through tail rotor strikes. The tail rotor is the largest cause of injury to ground personnel. The tail rotor also dominates the helicopter's acoustical signature and contributes substantially to the pilot's work load.
The concept of replacing the tail boom rotor on a helicopter with an air management system in an enlarged tail boom has been developed over the years, first by Hughes Helicopters, then by the Mc Donnell Douglas Helicopter Company, and, most recently, by the Boeing Company. The system has been employed on light helicopters and is commonly known by the trademark NOTAR.RTM..
The NOTAR system uses air from a controlled circulation system in an enlarged tailboom for anti-torque and directional control. Airflow within the boom is generated by a variable pitch axial flow fan mounted in the helicopter fuselage. The low pressure air is released through slots along the right side of the boom and also through a directional jet thruster at the end of the boom. The released air from the slots on the right side of the boom captures the downwash from the main rotor, creating lift laterally along the boom, the same as airflow over the top of a wing. In this case however, the "lift" is to the side. In present systems, about 60% of the anti-torque needed in a hover is achieved in this fashion. Directional control is achieved by venting more air through the thruster at the end of the tail boom and also by use of pilot-controlled vertical stabilizers located on the boom.
The proportion of the force produced by each of these two subsystems depends upon the mode of flight of the helicopter. In high speed translational flight, the main rotor wake trails the tail boom, and, as a result, the slots on the right side of the boom produce essentially no side force, and all of the necessary force is provided by the jet thruster. When the helicopter is in hover mode, the circulation control from the slots in the boom provides about 60% of the anti-torque force required while the jet thruster provides the balance. It can be appreciated that the air flow through the jet thruster accounts for most of the energy needed by the NOTAR yaw control system.
The system, in an earlier form, is described in much more detail in U.S. Pat. No. 4,200,252 to Logan et al. for a "Helicopter Antitorque System Using Circulation Control." A later embodiment is described in U.S. Pat. No. 4,948,068 to VanHorn et al. for "Circulation Control Slots in Helicopter Yaw Control System." Both of these patents are owned by the assignee of the present invention and are incorporated by reference in their entirety.
The thruster system presently in use works well but has a number of areas that could be improved. For example, the thruster performance is marginal in certain wind azimuth conditions. The current design only produces about 60% efficiency, chiefly due to the need to have separate turning vanes inside the thruster for both left and right yaw. Also, the force vector produced by the current design is inclined downwardly from the desired horizontal direction, thereby inducing yaw/pitch coupling. In addition, the current design employs an outer rotating thruster assembly which tends to "drum" against the inner fixed thruster assembly and tends to prematurely wear out the thruster rollers between the outer and inner thruster assemblies.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for an improved thruster design that will overcome the above-discussed drawbacks and shortcomings of the present thruster systems. This improved design should not be more costly than the present system and should deliver improved performance and maintenance costs.
As will become apparent hereinafter, the present invention fulfills this need in the art by reversing the inner and outer thruster assemblies such that the outer thruster assembly is in a fixed position with the inner thruster assembly now being the rotating assembly. Since the air deflecting vanes are within the inner thruster assembly, they can now be optimized for thrust in a single direction, greatly improving the efficiency of the system. Further improvements to the outer thruster eliminate the detrimental downward thrust component in the prior art systems, thereby decreasing pilot workload.